1. Field of the Invention
The present invention relates to a transmission control circuit for ON/OFF switching transmission waves in a TDMA (time division multiple access) system radio communication device, such as a portable telephone or a cordless telephone.
2. Description of the Related Art
In a TDMA communications system, the same frequency is used for both the transmission and reception signals. At the time of transmission, a modulated wave is output, and, at the time of reception, no modulated wave is output. That is, a modulated wave is ON/OFF switched for each interval of, for example, approximately 1 ms, thereby alternately repeating transmission and reception. As a means of this communications system, a transmission control circuit is used which is equipped with a driving voltage ON/OFF circuit which generates a modulated wave for both transmission and reception and ON/OFF-switches a +B voltage, which is the driving voltage for output amplification, for each interval of approximately 1 ms.
FIG. 3 shows such a conventional transmission control circuit, The transmission control circuit of FIG. 3 includes: a transmission circuit B4 consisting of high-frequency amplifier circuits A1 and A4 for amplifying a modulated wave F1; an antenna circuit A3 for switching between transmission and reception signals; a control section C for outputting an ON/OFF switching signal S; and a driving voltage ON/OFF circuit B2 for ON/OFF switching the driving voltage for the high-frequency amplifier circuits A1 and A4 on the basis of the ON/OFF switching signal S,
The driving voltage ON/OFF circuit B2 includes: an input terminal 3 to which the ON/OFF switching signal S is input; an unnecessary signal short-circuiting capacitor C2 which is connected between the input terminal 3 and the ground; a resistor R2 one end of which is connected to the input terminal 3; a PNP-type transistor TR1 whose base electrode is connected to the other end of the resistor R2, whose emitter electrode is connected to a +B power source terminal BT through a protective resistor R0, and whose collector electrode is grounded; a resistor R1 connecting the emitter electrode of the transistor TR1 to an output terminal 4; and a capacitor C1 connected between the output terminal 4 and the ground. The resistor R1 and the capacitor C1 form a low-pass filter.
The transmission circuit B4 includes: an input terminal 1 to which the modulated wave F1 is input; a first. high-frequency amplifier circuit A1 for amplifying the input modulated wave F1; a second high-frequency amplifier circuit A4 for amplifying the output of the first high-frequency amplifier circuit A1; and an output terminal 2 for emitting the output of the second high-frequency amplifier circuit A4. The driving voltage for the high-frequency amplifier circuits A1 and A4 is supplied from the driving voltage ON/OFF circuit B2.
Next, the operation of the driving voltage ON/OFF switching circuit B2 and of the transmission circuit B4 will be described. First, the ON/OFF switching signal S is output from the control section C to the driving voltage ON/OFF circuit B2, The ON/OFF switching signal S is a signal exhibiting a rectangular waveform in which high-level states of approximately 1 ms and low-level states of approximately 1 ms alternate with each other.
When the ON/OFF switching signal S is at the high level in the driving voltage ON/OFF circuit B2, the transistor TR1 is in the non-conducting state, and a voltage which is substantially equal to a voltage Vcc, applied to the +B power source terminal BT, is applied to the emitter electrode of the transistor, while, when the ON/OFF switching signal S is at the low level, the transistor TR1 is in the conducting state, and the voltage at the emitter electrode is approximately 0.
These emitter voltages are output to the output terminal 4 through the resistor R1 and applied to the two high-frequency amplifier circuits A1 and A4 of the transmission circuit B4 as a driving voltage S3. Thus, when the ON/OFF switching signal S is at the high level, that is, at the time of transmission, the high-frequency amplifier circuits A1 and A4 are supplied with the driving voltage S3 of approximately Vcc volts to be triggered into the operating state, and the modulated wave F1 is amplified and output to the output terminal 2. On the other hand, when the ON/OFF switching signal S is at the low level, that is, at the time of reception, the high-frequency amplifier circuits A1 and A4 are supplied with no driving voltage and are in the non-operating state, and the modulated wave F1 is not amplified and not output to the output terminal 2.
In the driving voltage ON/OFF circuit B2, the low-pass filter, which is formed by the resistor R1 and the capacitor C1, appropriately attenuates higher harmonics contained in the driving voltage S3 before adding them to the high-frequency amplifier circuits A1 and A4, thereby preventing the higher harmonics from becoming interference signals to interfere with communications between others.
Further, in the transmission circuit B4, both the first and second high-frequency amplifier circuits A1 and A4 consist of high-frequency amplifier circuits having a linear amplification characteristic and the same construction, whereby no higher harmonics are generated in the transmission circuit B4 and communications interference by higher harmonics acting as interference signals is avoided.
However, even when they are designed to be the same circuits, the two high-frequency amplifier circuits A1 and A4 may still involve some discrepancies due to a manufacturing dispersion, which may in some cases lead to generation of higher harmonics in the transmission circuit B4. For example, the case will be considered when the ON/OFF switching signal S is switched to change the device from the receiving state to the transmitting state, and the high-frequency amplifier circuits A1 and A4 are caused to rise by the driving voltage supplied thereto. If, in this condition, the rise characteristic of the high-frequency amplifier circuit A4 is such as can be represented by the solid line a in FIG. 4, and the rise characteristic of the high-frequency amplifier circuit A1 is such as can be represented by the broken line h in FIG. 4, which is steeper than the solid line a, a higher harmonic will be generated during the period T1 in which the high-frequency amplifier circuit A4 rises.
This is attributable to the fact that the modulated wave F1 output from the high-frequency amplifier circuit A1, which has risen earlier, is distorted by a non-linear amplifying action of the high-frequency amplifier circuit A4, which has not completely risen yet, with the result that a higher harmonic is generated.
Such a higher harmonic is also generated at the change from the transmitting state to the receiving state. That is, when, as shown in FIG. 4, the fall characteristic of the high-frequency amplifier circuit A4 is such as can be represented by the solid line e, and the fall characteristic of the high-frequency amplifier circuit A1 is such as can be represented by the broken line d, which is gentler than the solid line e, a higher harmonic will be generated during the period T2 in which the high-frequency amplifier circuit A4 falls, by an action similar to that at the time of rise.
Thus, when there is a difference in terms of rise and fall characteristics between the first and second high-frequency amplifier circuits A1 and A4, higher harmonics are liable to be generated at the rise and fall of these high-frequency amplifier circuits, and such higher harmonics may become interference signals to interfere with communications.
In the transmission control circuit, a lot of unnecessary waves are generated in the transmission circuit B4 at the time of rise or fall of the high-frequency amplifier circuits A1 and A4, so that the frequency band of the modulated wave is expanded at the time of transmission, with the result that mutual interference of frequencies occurs between adjacent channels (adjacent frequencies), thereby causing communications interference.
Further, in the conventional circuit configuration, it is necessary to adopt high-frequency amplifier circuits of a linear amplification characteristic for the two high-frequency amplifier circuits A1 and A4 as a means for mitigating the higher harmonics generation, which causes generation of unnecessary waves, resulting in a large current consumption. Thus, when driven with a battery, the transmission control circuit can operate for only a short period of time, so that it is not suited to be mounted on a small size portable communication device or the like.